OXIDATION, ALLOMERIZATIOX AND REDUCTION OF CHLOROPHYLL 1777 



complex, with a band shift from 645 to 610 m^u; in strong light, the process 

 was reversed and the band returned to 645 Tn/j,. 



Reactions of this type had been observed also by Rabinowitch and 

 Weiss; however, these reactions are quite different from the instantaneous 

 and completely reversible transformation to which the hypothesis of re- 

 versible oxidation was applied. Ashkinazi et al. loft chlorophyll and ferric 

 chloride to react in the dark for 24 hours before observing the spectral 

 change; according to Rabinowitch and Weiss, reversibility is lost after a 

 few minutes. It is quite likely that a chlorophyll solution left standing 

 for several hours with ferric chloride will be converted to pheophytin. 

 This conversion is essentially irreversible (except via the Grignard reaction). 

 Green pheophytin complexes of different divalent ions, Zn + +, Cu + "'", etc., 

 are well known; they can be formed by direct substitution, but not in- 

 stantaneously; and their absorption spectra differ markedly between them- 

 selves, and from that of the Mg + + derivative (chlorophyll). 



Watson (1953) confirmed the finding of Rabinowitch and Weiss that 

 the decoloration of chlorophyll a in methanol by ferric chloride is spectro- 

 scopically fully reversible by immediate addition of reducing salts, such as 

 CU2CI2, and therefore must be attributed to regeneration of the original 

 pigment rather than to the formation of new green compounds (such as 

 the metal complexes of pheophytin, suggested by Ashkinazi et al.). On 

 the other hand, the much slower restoration of the green color upon stand- 

 ing, or upon the addition of non-reducing salts, such as NaCl, leads to 

 "allomerized" chlorophyll, with its distinct spectrum; regeneration by 

 hydrociuinone results in a still different, unknown green compound. In- 

 terpretation of the reversible reaction wdth FeCls as oxidation is supported, 

 according to Watson, by analogy with the transitory bleaching of chloro- 

 phyll observed upon addition of bromine or iodine, and by the formation 

 of allomerized {i. e., oxidized) chlorophyll upon standing of the decolorized 

 solution. The acceleration of the transformation by non-oxidizing salts 

 may be similar to the known acceleration of allomerization by LaCl2; 

 dissolved oxygen is needed in the latter case as oxidant. 



Dunicz, Thomas, Van Pee and Livingston (1951) built a flow system 

 to study spectroscopically the intermediates of chlorophyll reaction with 

 alcoholic alkali {"phase test"). They found that the brown intermediate 

 [an ether, ionized either at C(10) or at the enol at C(9)] is formed without 

 the participation of oxygen, and is converted into a green chlorine by reac- 

 tion with oxygen. In the absence of oxygen, a slow irreversible reaction 

 ensues. Weller (1954) measured the absorption spectrum of the phase 

 test intermediate with much better precision than Dunicz et al. Its bands 

 were located (in pyridine) at 683, 524, -186, 428 and 375 m/x for chlorophyll 

 a, and at 630, 558, 560, 505 and 444 mu, for chlorophyll h (main bands itali- 



